Hi, I am Dave, I will help you with this.
Important - disconnect totally from the Internet
Open the Settings App (Gear icon on your Starr Menu)
Go to Update and Security - Activation
Click Change Product key
Enter this generic product key: DXG7C-N36C4-C4HTG-X4T3X-2YV77
When prompted, click 'Activate Product'
If the above does not work first time, restart your PC and try again.
Note the generic key will cause the Pro edition to upgrade to the Pro for Workstations edition, then your own legal licence will activate automatically after you restart (not shut down) your PC